Conventional electronic functional materials using the interaction between magnetic ions and electric conduction electrons in a solid include various magnetic memory elements and giant magnetoresistive elements. The magnetic states of those functional materials are the long range ordered states in which they are energetically stable under given circumstances.
When the interaction between the geometrical arrangement of the magnetic elements on the lattice sites and the magnetic moments (or spins) of the magnetic elements satisfies a certain condition, the spin arrangement is not determined unique but many states having the same energy level degenerate even in the proximity of the absolute zero temperature. This situation is termed as “geometrical frustration”.
In that case, though, the degenerated states separate again when an external effect, such as an external magnetic field, is applied. This means that the magnetic state of the crystal can be controlled by applying an external effect such as an external magnetic field. Also, in that case, it is known that other quantum effects such as an anomalous Hall effect due to a local magnetic field may occur.
On the other hand, the crystal field effect is also important due to the coupling of orbital angular momenta and spins of electrons. Owing to this effect, energy levels can be changed by a magnetic field, which means that the magnetic states of a crystal can be controlled.
In oxides having the pyrochlore structure as shown in FIG. 1, the three-dimensional network of corner-sharing tetrahedra (with an O atom at the corner) causes geometrical frustration when magnetic elements R exist at the corners. When rare earth elements are used in this case, the crystal field effect is also important. That is, both the geometrical frustration and the crystal field effect characterize the system.
In some oxides having the pyrochlore structure, the geometrical frustration is also called as “spin ice”, because of the analogy in the spin arrangement of oxygen-magnetic ion system to the spatial arrangement of the oxygen hydrogen system in water ice (M. J. Harris et al., Phys. Rev. Lett. 79, 2554-2557 (1997)).
Known compounds of such kind include Ti pyrochlore oxides (e.g., Ho2Ti2O7: M. J. Harris et al., ibid; Dy2Ti2O7: A. R. Ramirez, Nature 399, 333-335 (1999)) and Sn pyrochlore oxides. But these oxides are insulators, so that the practical use or application to electrical functional elements are quite limited.
Among Mo- (e.g., Y2Mo2O7: M. J. P. Gingras et al., Phys. Rev. Lett. 78, 947-950 (1997)), Mn- and Ru-pyrochlore oxides, some are electrically conductive, but they develop well known magnetically ordered states such as the spin-glass ordering or antiferromagnetic ordering due to disorders contained in those materials or due to the structural phase transition, so that the large specific heat that should develop when the geometrical frustration exists does not appear.
As explained above, the state containing the geometrical frustration can be called as a “magnetic state containing controllability” in the sense that the magnetic state can be controlled by applying an external magnetic field. In that case, though, its application to industrial use is practically quite limited if it is not associated with some device for controlling it or with some sensor for detecting the magnetic state. Thus, for the purpose of industrial application, it is desired to develop a material which can show the spin ice state, or similar state, and has a good electrical conductivity.
It is another advantage that such a material that develops no magnetic transition to long range ordered magnetic state down to low temperatures has a large specific heat in a wide range of temperature. When such a material is intended to be used for a thermal storage material, it is strongly desired to have a large thermal conductivity to exchange heat with peripheral devices. From this point also it is desired to have a good conductivity or to be metal, rather than an insulator.